1. Field of the Invention
The present invention relates to motor servo circuits for disc reproduction apparatus, and more particularly, to a motor servo circuit controlling mainly the phase of a spindle motor used for rotating a disc in a reproduction apparatus of optical discs such as laser discs (referred to as LD hereinafter) and compact discs (referred to as CD hereinafter).
2. Description of the Background Art
A conventional reproduction- apparatus of an optical disc such as a LD and a CD is provided with a motor servo circuit for servo controlling the phase and speed of a spindle motor for the rotation driving of an optical disc. FIG. 1 is a block diagram schematically showing a structure of such a motor servo circuit in a laser disc player (LD player) as an example of an optical disc reproduction apparatus.
Referring to FIG. 1, a LD1 is loaded on a chucking device 4 constituted by a turntable and a clamper provided at the top end of a spindle 3 of a spindle motor 2 at the time of reproduction of LD. LD1 is held by chucking device 4 and rotated. The information recorded on the signal face of LD1 is read by an optical pickup 5 to be supplied to a signal processing circuit not shown.
Spindle motor 2 is provided with a FG pulse generator not shown. This FG pulse generator generates a FG pulse signal having a frequency proportional to the rotating speed of spindle motor 2. The FG pulse signal is supplied to one input terminal 10 of a speed comparison circuit 9. The other input terminal 11 is supplied with a reference FG signal REFFG of a reference frequency provided from an oscillation circuit not shown.
At the start of the reproduction operation of LD1, spindle motor 2 is actuated by a speed error signal provided from speed comparison circuit 9. More specifically, speed comparison circuit 9 compares the frequency of the FG signal supplied to input terminal 10 with that of the REFFG signal supplied to input terminal 11 to generate a speed error signal having a pulse width proportional to the difference therebetween. The speed error signal is applied to a motor driving circuit 16 via a switch circuit 12 connected to the side of contact 12b at the initial state of the rotation drive of spindle motor 2 and via an equalizer amplifier 13 that will be explained later on. Motor driving circuit 16 is responsive to the output of equalizer amplifier 13 to actuate the rotation of spindle motor 2.
When the rotating speed of spindle motor 2 approaches a predetermined rotating speed by the rotation drive of motor driving circuit 16, the rotating speed of spindle motor 2 is captured to the aforementioned predetermined speed by a speed servo according to the aforementioned speed error signal.
When the rotating speed of spindle motor 2 started to be captured to a predetermined speed, a condition wherein the recorded information can be read out from the signal face of LD1 is established. The horizontal synchronizing signal in the video signal read out by optical pickup 5, i.e. the PBH signal is separated by a signal processing circuit not shown and supplied to one input terminal 7 of phase comparison circuit 6. The other input terminal 8 is supplied with a reference horizontal synchronizing signal REFH having a reference phase provided from an oscillation circuit not shown. Phase comparison circuit 6 compares the phase of the PBH signal supplied to input terminal 7 with that of the REFH signal supplied to input terminal 8 to generate a phase error signal having a pulse width proportional to the magnitude of shift of the phase of the PBH signal from the reference phase.
The PBH signal separated at the signal processing circuit not shown is supplied to a lock detecting circuit 57. Lock detecting circuit 57 detects the period of the PBH signal to generate a speed lock detection signal a for providing the same to the control input of switch circuit 12, when the detected period is within .+-.20% range of a predetermined period of 63.5 micro seconds. Switch circuit 12 responds to the speed lock detection signal a to be switched to the side of contact 12a.
As a result, the phase error signal provided from phase comparison circuit 6 is supplied to equalizer amplifier 13 via switch circuit 12. Equalizer amplifier 13 is constituted by an integration and amplifying circuit 14, and a phase compensation circuit 15. Integration and amplifying circuit 14 implements a low pass filter processing for the applied phase error signal, and amplifies the obtained signal. Phase compensation circuit 15 functions to compensate for phase delay by 90.degree. of the phase error signal associated with the low pass filter processing by integration and amplifying circuit 14.
The phase error signal from equalizer amplifier 13 is supplied to motor driving circuit 16. In response, motor driving circuit 16 drives the rotation of spindle motor 2 to capture the rotation phase of spindle motor 2 to a reference phase.
At the time of LD reproduction, there is periodical time base variation in the rotation of the LD caused by eccentricity of the LD itself, or that of the chucking device of the LD player. Therefore, circuit constants of the above mentioned equalizer amplifier 13 are set to values so that the servo characteristics of the entire phase servo circuit constituted by phase comparison circuit 6, equalizer amplifier 13, motor driving circuit 16 and spindle motor 2 can capture the rotation phase of spindle motor 2 to a reference phase and suppress periodical time base variation of the LD rotation caused by various eccentricity.
The magnitude of eccentricity (eccentricity amount) of the above mentioned disc rotation differs greatly depending on the reproduced discs and is not uniform. It is therefore desired that various servo characteristics of the phase servo circuit are prepared so as to conform to the wide range of eccentricity amount. However, this is difficult to realize due to limitations on circuit configuration. In conventional practice, the circuit constants of equalizer amplifier 13 were selected to implement constant servo characteristics suitable for suppressing intermediate servo characteristics between servo characteristics of great eccentricity amount of rotation and that of a small eccentricity amount, i.e. a standard magnitude of eccentricity.
If the actual eccentricity amount of the rotation of the disc is greatly shifted from this standard eccentricity amount, the capture of the rotation phase of spindle motor 2 to a reference phase becomes difficult with the phase servo characteristics set in advance to deteriorate the accuracy of phase servo control. If it becomes difficult to suppress the time base variation of rotation, the time base variation of the reproduced video signal will not come within the possible correction range of a time base correction (TBC) circuit (not shown) provided for subjecting the reproduced video signal to time base correction. This means that accurate signal processing of the reproduced video signal is not always possible.
This problem is particularly significant in two side reproduction type LD players. Such a two-side reproduction type LD player is disclosed in U.S. Pat. No. 4,839,881, for example, wherein an optical pickup reads out information in the signal face of the lower side of the LD mounted on a chucking device, and then moves to the upper side of the LD to read out information on the signal face thereof.
The eccentricity amount at the time of lower side face reproduction and upper side face reproduction differ due to reasons such as the lower and upper sides of the LD not accurately bonded together in a two-side reproduction type LD player. Therefore, the phase servo characteristics realized to suit the eccentricity amount of the lower side reproduction do not always suit the eccentricity amount of the upper side reproduction. In such a case, time base correction of the reproduced signal from the signal face of the upper side could not be carried out accurately to result in a disturbed reproduced picture.
FIG. 2 is a block diagram showing another example of a conventional motor servo circuit for a LD player. Speed comparison circuit 9, switch circuit 12 and lock detecting circuit 57 shown in FIG. 1 are not depicted in FIG. 2. The phase servo circuit of FIG. 2 differs from the phase servo circuit of FIG. 1 in that the former comprises a gain switching circuit 18 provided between equalizer amplifier 13 and motor driving circuit 16, and a system control circuit 19 implemented by a microcomputer for controlling gain switching circuit 18.
The basic operations of the phase servo circuit of FIG. 2 is similar to that of the phase servo circuit of FIG. 1, except for the output of equalizer amplifier 13 (referred to as the first phase error signal hereinafter) supplied to gain switching circuit 18, whereby the level of the first phase error signal varies according to the type of the reproduced LD. The output of gain switching circuit 18 (referred to as the second phase error signal hereinafter) is supplied to motor driving circuit 16 to implement capture of the rotation phase of spindle motor 2 to a reference phase.
The inertia force required for the rotation of spindle motor 2 is different for a LD having a diameter of 20 cm and a LD having a diameter of 30 cm. If phase servo control is carried out with the same servo gain in all cases according to just the first speed error signal, an appropriate phase servo control may not be obtained due to the servo gain being too high or too low. A status signal (referred to as STAS signal hereinafter) for switching the servo gain is supplied to gain switching circuit 18 from system control circuit 19 according to the type of the reproduced disc, whereby the level of the phase error signal, i.e. the phase servo gain is switched accordingly.
FIG. 3A is a circuit diagram showing an example of a gain switching circuit 18. Gain switching circuit 18 of FIG. 3A is constituted by a discrete circuit comprising dividing resistors Ra and Rb for level adjustment, and an analog switch SW turned on/off by STAS signal. Corresponding to the type of the LD to be reproduced, analog switch SW of gain switching circuit 18 is turned on/off according to STAS signal from system control circuit 19 to change the level of the first phase error signal, whereby the first error signal is converted into a second phase error signal.
FIG. 3B is a circuit diagram showing another example of gain switching circuit 18. Instead of analog switch SW of FIG. 3A, the circuit of FIG. 3B employs a switch circuit formed of transistors Q and Qz, a diode D , bias resistors Rc, Rd, Re, and a bias voltage supply terminal Vcc.
Although the examples of FIGS. 3A and 3B have the servo gain switched into two stages, it is possible to switch the servo gain into three or more stages. In this case, a multi-contact analog switch SW may be implemented using a digital signal of a plurality of bits as STAS signal.
The conventional phase servo control circuit of FIG. 2 requires dedicated gain switching circuits of discrete configuration of a scale according to the number of stages of switching, in order to switch the servo gain according to the type of the disc. If there are many switching stages, the number of components of the gain switching circuit increases, to result in a phase servo circuit of high cost.